The invention relates to a method for the electrochemical processing of HCl gas to high-purity chlorine in which HCl gas-phase electrolysis is combined with hydrochloric acid electrolysis, with or without a consumable oxygen cathode, to form an HCl recycling unit.
HCl gas, a by-product of numerous different chemical reactions, is commonly converted to chlorine gas which can then be passed back into the production cycle again. In one known such method HCl gas is converted to chlorine gas by gas-phase electrolysis. The resulting chlorine gas, however, always still contains a certain amount of HCl gas which has not undergone fall conversion. The HCl gas is separated from the chlorine gas by technology which is known in principle. In the course of high-pressure liquefaction, cooling results in the production not only of the purified liquid chlorine but also of the HCl gas as xe2x80x9coverhead gasxe2x80x9d. High-pressure distillation of the liquid chlorine expels further residues of HCl gas from the chlorine. The gas-phase electrolysis of hydrogen chloride to chlorine gas is described, for example, in the patent specification U.S. Pat. No. 5,411,641.
The known separation method described above has the disadvantage that owing to the cooling and reheating operations the separation of the HCl gas from the chlorine is highly energy-intensive and complicated and that the separation of HCl is incomplete. In addition, it is impossible to avoid handling relatively large amounts of liquid chlorine in order actually to be able to start the process which results in hydrogen chloride as a by-product. The alternativexe2x80x94a hydrogen chloride liquid gas storage facilityxe2x80x94is even less attractive, since in that case even higher pressures must be employed in order to liquefy the HCl gas.
EP 785 294 A1 describes an electrolytic method in which aqueous hydrochloric acid having a maximum concentration of 20% is reacted anodically to chlorine gas in a membrane electrolysis cell. This reaction is operated cathodically with a consumable oxygen cathode. The current density which can be achieved in continued operation of this method is limited to about 4 kA/m2.
On the basis of the above-described prior art, therefore, there is a need for a simple method of processing HCl gas to elemental chlorine which avoids the complicated liquefaction of HCl and/or chlorine, can be operated with a considerably higher current density, is more favourable in terms of energy consumption, and, moreover, can easily be coupled onto any other continuous processes where HCl gas is obtained as a product.
The object is achieved in accordance with the invention by a method for the electrochemical processing of HCl gas to high-purity chlorine which forms the subject of the invention and is characterized
in that the HCl gas is reacted in a first gas-phase electrolysis stage to give a chlorine gas which comprises HCl gas,
in that the HCl-comprising chlorine gas is scrubbed extractively in a second stage of a with depleted liquid hydrochloric acid as the absorbent, to form enriched hydrochloric acid, so freeing the chlorine gas from the HCl gas,
in that the enriched hydrochloric acid is broken down in a third stage of liquid-phase electrolysis into chlorine gas, with formation of depleted hydrochloric acid, and, especially when using a consumable oxygen cathode, to form chlorine gas and water,
in that the depleted hydrochloric acid is passed back into the absorption stage
and in that the chlorine streams generated in the gas-phase electrolysis stage and in the liquid-phase electrolysis stage are drawn off, combined if desired and dried if desired.
The chlorine streams can subsequently be passed again to a chlorine-consuming chemical process.